This invention relates to a method for the preparation of vinyl chloride polymers or copolymers. In particular, the invention relates to an improvement in polymerization used to prepare vinyl chloride polymers or copolymers having a relatively low molecular weight or degree of polymerization.
Low molecular weight polyvinyl chloride resins can be produced by polymerization at relatively high temperatures. However, the conventional high-temperature polymerization method is undesirable because of an extended gellation time of the resin, inferior flow of the resin in the molding, and increased amounts of residual vinyl chloride monomer absorbed in the resin. In order to avoid these drawbacks, it has been proposed that certain chain transfer agents be added to the polymerization mixture. The chain transfer agents that are suitable for this purpose are, for example, saturated hydrocarbons, such as n-pentane and n-hexane; saturated or unsaturated chlorinated hydrocarbons, such as carbon tetrachloride, trichloroethylene and perchloroethylene; aldehydes, such as propionaldehyde and n-butylraldehyde; and certain mercapto-containing organic compounds, such as dodecyl mercaptan.
The above-mentioned chain transfer agents all have their respective defects. The saturated hydrocarbons are used in large quantities; about 8 to 10% by weight based on the amount of vinyl chloride monomer used. They produce monomer in order to produce vinyl chloride polymers having an average polymerization degree as low as about 700 by suspension polymerization at about 60.degree. C. Hydrocarbons tend to be emitted into the atmosphere from the polymer products which are subjected to post-polymerization processes, or are released from the finished resin products during the course of storage, bringing about the problem of environmental pollution.
Other chain transfer agents are the saturated or unsaturated chlorinated hydrocarbons; they may be used in a reduced amount, for example, from 0.7 to 1.0% by weight based on the amount of vinyl chloride monomer used. However, the problems of environmental pollution and the detrimental effects on the human health cannot be solved due to the inherent toxicity of these compounds.
The aldehyde compounds used as chain transfer agents are eventually decomposed by heat in the course of a polymerization run, and the decomposition products exert a retarding effect on the velocity of the polymerization reaction and also exhibit toxic effects on the human body. The amount used may be reduced to 0.2 to 0.5% by weight based on the amount of vinyl chloride monomer used.
There are also several known methods in which certain mercapto-containing organic compounds are employed to control the molecular weight of polyvinyl chloride resins. For example, dodecyl mercaptan is used in the emulsion polymerization of various kinds of vinyl monomers, as disclosed in Japanese Patent Disclosure No. 50-32281; several kinds of alkyl thioglycolates are used in the polymerization of vinyl chloride, as disclosed in Japanese Patent Publication No. 49-31746; and several kinds of mercapto-containing organic compounds having 4 or less carbon atoms per molecule are used in low-temperature bulk polymerization of vinyl chloride in a continuous process, as disclosed in German Patent OLS No. 2046143. These prior art methods using mercapto-containing organic compounds are defective in that they require large amounts of the compounds which eventually lead to retardation of the polymerization velocity and coloring of the resultant polyvinyl chloride resin, as well as unpleasant odors inherent in the mercapto-compound.